An Infrared Spectroscopic Study on Phase Behavior of Sodium Sulfonate-Water System

Fourier transform infrared spectroscopy has been used to study the thermotropic phase transitions of 20% sodium n-dodecyl sulfonate-water system.Two kinds of coagel phase（opaque and microscopically inhomogenious system with hydrated crystals and water ） with different thermal hystereses were prepared for the SDS-water system.The C-H stretching, CH₂ scissoring and SO₃^- stretching bands of SDS and the O-H stretching band of water are discussed in each phase.
The temperature dependence of the bands show that both unannealed and annealed samples exhibit the phase transition from coagel to liquid crystal at 47℃ and the reverse transition around 35℃. Although the main disinction between the two kinds of coagel phase is difference of the packing form of alkyl chain,the alkyl chains are in all-trans conformation and the internal rotation of the terminal CH₃ group is highly restricted in both coagel phase.In the liquid crystal phase,the alkyl chains are in disordered liquid-like state with the collapse of transzig-zag planes and free rotation of the methyl groups.The spectra of sample in both coagel phases below 0℃ exhibit the ice band originated from free water above 0℃,The shoulder band originated from the bound water can be also distinguished from the band of free water.The states of the hydrophilic sulfonic group are closely related to the states of water molecule in different phases.The spectra also indicated that the packing of alkyl chain is strongly affected by the evaporation of water.